On heating, numerous compounds are not converted from the crystalline state with defined short-range and long-range order of the molecules directly into the liquid, unordered state, but rather pass through a liquid-crystalline phase in which the molecules are mobile but the molecular axes form an ordered structure. In this case, stretched molecules often form nematic liquid-crystalline phases which are characterized by long-range order of orientation by virtue of parallel arrangement of the longitudinal axes of the molecules. When such a nematic phase comprises chiral compounds or chiral molecular moieties, a chiral nematic or cholesteric phase can form, which is characterized by a helical superstructure. The lesser or greater the proportion of chiral compound or chiral molecular moiety in a given system, the greater or lesser is the pitch of the helical superstructure. In order that electromagnetic radiation of comparatively long wavelength, for example in the region of visible light or NIR radiation, can be reflected to a sufficient degree, the formation of maximum layer thicknesses of the chiral-nematic phase is required, which, however, is typically associated with an increase in the misorientation of the helical superstructure.
A cholesteric pitch p is defined as distance along the helical axis for 360° twist. The helical superstructure involves a change of refractive index from layer to layer and gives thus rise to interference effects. When ordinary white light is incident normally on a film of a cholesteric material with the helical axis perpendicular to the substrate, selective reflection of a finite wavelength band occurs in a manner analogous to the Bragg reflection. The reflected band is centered about a wavelength λ0, which is related to the helical pitch length p of the phase and its average refractive index n by λ0=np. The light reflected is circularly polarized with the same sense of polarisation as the helical sense of the liquid crystal phase. Light circularly polarised with the opposite handedness is transmitted through the sample together with those wavelengths of not being reflected. As a result of the anisotropy of the system light experiences double refraction. The band width of the selectively reflected band is given by Δλ=p Δn. The angular dependence for an incidence and observation angle θ is given by λq=λ0 cos θ, A cholesteric film thus shows a strong angle dependent color travel.
Owing to their remarkable optical properties, liquid-crystalline materials, especially nematic, chiral nematic or cholesteric materials, are of interest in optical or electrooptical applications among others. However, the temperature range in which the liquid-crystalline phase occurs is often outside the desired application temperature or it extends only over a small temperature interval.
Principals of the liquid crystal phase (mesophase) are, for example, outlined in G. W. Gray, P. A. Winsor, Liquid Crystals and Plastic Crystals, Ellis Horwood Limited, Chichester, 1974.
The chiral molecular moiety may be present either in the liquid-crystalline molecule itself or be added as a dopant to the nematic phase, which induces the chiral nematic phase. This phenomenon was investigated first in cholesterol derivatives (for example H. Baessler, M. M. Labes, J. Chem. Phys. 52, 631 (1970)). By changing the concentration of a chiral dopant, the pitch and hence the wavelength region of selectively reflected radiation of a chiral nematic layer can be varied.
When the intention is to fix the liquid-crystalline ordered structures in the solid state, there are various possibilities. In addition to glasslike solidification in the course of cooling from the liquid-crystalline state, there is the possibility of polymerization into polymeric networks or, in the case that the liquid-crystalline compounds comprise polymerizable groups, of polymerizing the liquid-crystalline compounds themselves.
Furthermore, maximum refraction of the liquid-crystalline materials and maximum birefringence of liquid-crystalline materials is often desirable.
By oligomerizing or polymerizing a polymerizable liquid-crystalline composition, it is possible to prepare oligomers or polymers which may in particular also be obtained in the form of a film, i.e. of a self-supporting layer of uniform thickness. This film may be disposed on such a substrate that suitable measures make possible easy removal and transfer to another substrate for permanent disposition. Such a film can be used, for example, in the field of film coating and in laminating processes.
Furthermore, such films, whose properties have been adapted to the particular end use, can be used in a wide variety of fields.
It is additionally possible to coat or print substrates by means of a polymerizable liquid-crystalline composition, by applying this composition to the substrate and subsequently polymerizing it.
With regard to the procedure for printing or coating substrates with liquid-crystalline materials, reference is made mutatis mutandis to the document WO 96/02597 A2. Furthermore, a polymerized layer which has been produced with the aid of such a procedure and partly or fully covers the original substrate surface should also be considered as a substrate, and so the production of multiply printed and/or coated substrates is also possible.